Electroconvection in smectic C liquid-crystal films visualized by optical anisotropy

Electrohydrodynamic convection due to the Carr-Helfrich (CH) effect has been extensively investigated in nematics. It has been predicted also for freely suspended smectic C and $\rm C^*$ films. On the other hand, in smectic A films another type of electroconvection (vortex flow) has been described (Morris S. W., De Bruyn J. R. and May A., Phys. Rev. Lett. 65 (1990) 2378) but is not sufficiently understood by theory so far. We report experiments on freestanding smectic C films in lateral AC electric fields. A convective instability bifurcating continuously from the homogeneous ground state is found. Evidently the mechanism of the instability is of a similar type as in ${\rm S}_{\rm A}$. The director field in the convection vortices is directly visualized in the optical texture.